Proton-transfer reactions of two systems, ionization of a series of small carbon acids in water (the Pearson system) and reactions of substituted phenylnitromethanes, were examined in detail computationally. Comparison of experimental reactivity and pK(a) with calculated relative activation barrier and reaction energy for the Pearson system suggested that the origin of the well-know nitroalkane anomaly does not reside in the reactivity but in the equilibrium. For the reactions of substituted phenylnitromethanes, proton transfers among three species, PhCH(2)NO(2), PhCHNO(2)(-), and PhCH=NO(2)H, were examined, and the role of the aci-nitro species (PhCH=NO(2)H) was evaluated on the basis of its stability and reactivity. Protonation of PhCHNO(2)(-) by H(2)O was suggested to occur kinetically on the oxygen site, but due to its instability PhCH=NO(2)H does not contribute to the overall reaction energetics. The protonation of PhCHNO(2)(-) under acidic conditions occurs on the oxygen site to give PhCH=NO(2)H both kinetically and thermodynamically. The aci-nitro species thus formed appears to give PhCH(2)NO(2) via intramolecular H(2)O-mediated proton transfer, but a possibility of the route through PhCHNO(2)(-)-C-protonation would not be fully eliminated.